Add Modernizer feature detection and flexbox directory to the site

@benbarnett @aduggin 

As per discussion with Ben I've had a go at adding feature detection for flexbox (and svg) and re-implementing flexbox. I've tested it in the troublesome mobile browsers and works perfectly.

I'm not sure the correct way to have Modernizer load and whether it needs further performance optimisation...discuss?
Would be good to not introduce an blocking http request in the head. Would be good to investigate adding it inline.